Peterborough, N.H. - Circuits built from high-critical-temperature superconductors might support quantum computing, according to experiments performed by physicists at Chalmers University of Technology (Goteborg, Sweden). Working with a group at Italy's University of L'Aquila, the physicists have directly observed macroscopic quantum effects in high-critical-temperature Josephson junctions.
Theory rules out any observable separation of energy levels in such structures, because of energy dissipation at the lowest level, so the researchers' results were unexpected. In light of the contradiction, the theory on how high-Tc junctions operate may need to be revisited.
But apart from the interesting physics involved, the results indicate that energy levels in high-Tc junctions can be used to represent quantum bits (qubits). Even more intriguing is that a series of such bits would have long coherence times. That could make quantum processor designs with high-Tc circuits feasible.
In a report on the work in the Jan. 6 edition of Science, the researchers said they built the device by interfacing two YBCO ceramic superconductor samples that had different crystal orientations, producing a thin insulating region from the grain boundaries at the interface. The junction's particular geometry generated a stray capacitance in the strontium titanate substrate, and that effect turned out to dominate the capacitance of the system. (Josephson junctions typically have a small capacitive region-a thin tunnel junction interrupting a superconducting ring.)
The large stray capacitance produced a separation of the energy levels in the device, critical for demonstrating the macroscopic quantum effects needed for quantum computing. In essence, the larger capacitance protects the quantized energy levels from external effects that lead to decoherence, the researchers said.
Previous work has shown that low-critical-temperature Josephson junctions can achieve the same quantization of energy levels and insulation from external environmental effects, and a number of groups have sought to build quantum computers using low-Tc junctions. The current breakthrough may make it possible to do the same thing in a system operating at the liquid-nitrogen temperature rather than at the liquid-helium temperature required by conventional low-critical-temperature junctions.